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Discussion Starter · #1 ·
I'm having a problem with my new Panasonic PTL-C75U causing hum in my audio.


The projector is the only device in my setup with a three-prong plug. Everything else uses polarized two-prongers, all plugged in to the same power strip (single point ground).


I'm using the analog outs of the DVD player because my receiver doesn't have an AC-3/DTS decoder.


I've tried both my old Sony interlaced player & new Panasonic RP-56 & they both exhibit the problem. Neither do connected to a tube-TV via s-video.


If I connect only the component cable (25' Canare from a2z) to the "RGB1" jack or a 6' s-video cable, no hum. If I only plug in the projector, but don't plug in a video source, no hum. I've beeped out (continuity tested) the cables to insure no broken conductors.


This happens even if the projector is in standby: main power switch on, but bulb & fan off.


The hum occurs in the background of the audio (-30dB) from the DVD, but is strong enough (& my receiver leaky enough) that other sources (e.g. TiVo) have a very weak hum (-80dB). [dB figures are guesstimates]


So.... ground loop right? The projector's ground prong is connected to the jack's ground. The other components' jack grounds are all connected to their chassis grounds but not to either side of the AC plug. If the DVD player were 3 prong grounded, there would be a clear loop: video cable ground, video out jack ground through DVD chassis, AC ground, through powerstrip, AC ground through pj chassis, video in jack.


However, both DVD players and the receiver don't have AC grounds.


I haven't tried lifting (disconnecting) the projector's AC-ground yet, but that's next. I also need to try to measure the amplitude of the noise signal.


I know most people will use the digital audio connection and a plastic fiber would prevent noise from leaking into the receiver, but a coax connection could conduct noise into the ground plane of the receiver, possibly leaking into the analog side.


---


I've tested every outlet in the house for improper hot/neutral/ground assignments & they came up clean. However, the AC outlet in question *is* a long way from the whole-house grounding point.
 

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Did you try disconnecting your cable or sat coax? I spent alot of time reconnecting components before I unplugged the cable. Viola. That's the source of probably 80% of hum issues out there. I bought a cable isolator from Polk Audio ($50) and it cured the hum issue completely. Though it did also reduce pic quality from cable tv.
 

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Discussion Starter · #3 ·
Quote:
Originally posted by HTCrazy
Did you try disconnecting your cable or sat coax? I spent alot of time reconnecting components before I unplugged the cable. Viola. That's the source of probably 80% of hum issues out there. I bought a cable isolator from Polk Audio ($50) and it cured the hum issue completely. Though it did also reduce pic quality from cable tv.
I was dubious, but you were right: it was the cable TV feed.


The ground loop path seems to be cable-TV cable thru cable box to cable box audio out to receiver, thru receiver to DVD audio input thru DVD to video output to projector's video-in thru projector to AC ground.


The last (very long) segment of the loop is from AC ground to the cable-TV ground.


The splitter block on the outside of the house was connected to a ground wire, but I'll have to climb underneath the house to see how good that ground is.


I tried bypassing two splitters with direct connections & removing the cable-TV ground, but the hum remains.


When the projector is connected to the cable box's video out instead of the DVD player's, the hum goes away. I think the loop remains, but now it goes right from cable box to projector & doesn't go through the receiver, so it doesn't get into the audio. Connecting both the cable box and DVD player or simply providing a ground connection between the cable box and projector attenuates the hum.


My cable-TV signal is only fair quality as it is, so I don't think the Polk device is for me.


I guess I'll ground the cable box to AC ground to keep noise down for casual use and pull the cable box's audio outputs for serious DVD-projector watching. I'll also look into improving the cable-TV ground where it comes down from the pole.
 

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I have the same issue. Mine occurs when I have the audio out connected from the cable box to the receiver and the RF out to the TV. Since the TV audio out to the receiver (i.e.-using the TV to decode the RF) doesn't cause an issue, I'm wondering if it isn't the craptastic cable box. Maybe poor grounding inside? Or an uber-cheap decoder?


You might want to check out "hum eliminators" or ground-loop isolators from any number of companies. Decent ones start around $120 and go up from there. I'd recommend trying a local music store.
 

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Discussion Starter · #5 ·
The problem isn't a poor component anywhere but just a conflict between grounding philosophies. All of my equipment has signal ground floating relative to earth ground and power from a 2-prong jack *except* the new piece: the projector.


If everything (esp. the cable box) used 3-prong jacks, there would be a bunch more short ground loops (there are already ground loops between things like the left & right shields in any audio connection), but no long ones. The longer the loop, the higher the potential (i.e. voltage) that can accumulate.


A connection with inifinitesmal ohms with a continuity tester is only well-connected at DC. At 180Hz (3rd harmonic of 60Hz), it might be a lot worse.


If I ground the cable run's shield to AC/earth at both ends, I should kill it's ability to act as an antenna.


I forgot to mention that I did lift the projector's gound & that killed the hum. I believe that to be eliminating the difference in grounding methods (everything floats) in the wrong direction.
 
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